Technical Field
The present invention relates to an NMR (Nuclear Magnetic Resonance) probe, and in particular, to a technique for changing a characteristic of an electronic circuit in an NMR probe.
Related Art
In nuclear magnetic resonance (NMR) apparatuses, an NMR probe (an NMR signal detection probe) is placed along with a sample within a superconductive magnet that generates a static magnetic field. The NMR probe has a transmission and reception coil. The transmission and reception coil has functions to apply a varying RF (radio frequency) magnetic field to the sample during transmission, and to detect an NMR signal of the sample during reception. Because a resonance frequency differs depending on a nuclear species to be observed, in a sample measurement, a high-frequency signal having a frequency adapted to the nuclear species to be observed is supplied to the transmission and reception coil. In general, a detection circuit in the NMR probe includes, in addition to the transmission and reception coil, a variable capacitor for tuning and a variable capacitor for matching. In other words, the detection circuit has a tuning circuit and a matching circuit.
Prior to the sample measurement, tuning for the detection circuit (adjustment of operation conditions) is executed. In other words, the tuning and matching are executed. In order to achieve tuning and matching, a directional coupler is provided on a signal path from the transmission side to the detection circuit, and a reflected wave returning from the detection circuit to the transmission side is observed. A voltage level of the reflected wave changes according to a degree of detuning and a degree of mismatch. Therefore, by changing setting values (capacitances) of the tuning variable capacitor and the matching variable capacitor while referring to the voltage level of the reflected wave, the resonance frequency of the detection circuit is adapted to the resonance frequency corresponding to the nuclear species to be observed, and an impedance of the detection circuit is matched with an impedance at the transmission side.
A frequency range in which the tuning and matching of the detection circuit can be optimized (tuning range) is limited by a range of the capacitance of the variable capacitor, and the nuclear species that can be observed are also limited to nuclear species having resonance frequencies within the tuning range. For example, in a detection circuit having a tuning range of 100 MHz-120 MHz, only nuclear species having resonance frequencies in the frequency range can be measured.
As a technique for changing the tuning range of the detection circuit, U.S. Pat. No. 7,701,219 B discloses a capacitor switch in which a plurality of capacitors having different capacitances are fixed (refer to FIGS. 3A-3C and 4A-4H, and TABLE 1 of this reference). The capacitor switch is provided within the NMR probe, and the capacitor to be connected to the detection circuit can be switched by sliding the capacitor switch. With this process, the capacitance of the capacitor included in the detection circuit is changed, and, as a result, the tuning range is changed.
In the method described in U.S. Pat. No. 7,701,219, because the tuning range is changed using a limited number of capacitors, the tuning ranges that can be handled are limited. For example, a problem may arise in that the tuning range cannot be changed continuously, and the tuning range is discretely set. When the observation target is an organic substance, the number of nuclear species to be observed is not large. Therefore, by preparing capacitors in numbers corresponding to the number of nuclear species, it becomes possible to set a tuning range corresponding to the nuclear species to be observed. However, when the number of nuclear species to be observed is increased, the method described in U.S. Pat. No. 7,701,219 cannot handle the large number of nuclear species. For example, the number of nuclear species is large for inorganic substances, and thus, with the method of switching among a limited number of capacitors, it may not be possible to set a tuning range corresponding to the nuclear species to be observed. In addition, depending on a dielectric constant of the sample, the tuning range may differ for the same nuclear species. In order to handle such cases, a configuration may be considered in which the number of capacitors that may be switched is increased in the capacitor switch described in U.S. Pat. No. 7,701,219. However, a problem arises in that, as the number of capacitors is increased, the size of the NMR probe housing the capacitors is also increased. In addition, in order to handle a larger number of nuclear species, the number of capacitors to be provided becomes enormous, which is not practical. As described, with the method of using a limited number of capacitors, it is difficult to set a tuning range corresponding to an arbitrary nuclear species.
As described, it is desirable that the tuning range can be more flexibly changed corresponding to the nuclear species to be observed. In addition, it is also desirable that, in addition to the tuning range, the characteristics of the electronic circuit provided in the NMR probe can be flexibly changed. During this process, it is desirable that an electronic circuit characteristic such as the tuning range be changed while maintaining a peripheral environment of the electronic circuit provided in the NMR probe. For example, for a cool-type NMR probe, it is desirable that the electronic circuit characteristic be changed while maintaining a cooled state of the peripheral environment of the electronic circuit. Alternatively, when the inside of the NMR probe is maintained at vacuum, it is desirable that the electronic circuit characteristic be changed while maintaining the peripheral environment; that is, the vacuum, of the electronic circuit.
An advantage of the present invention is that, while maintaining a peripheral environment of an electronic circuit provided in an NMR probe as much as possible, a characteristic of the electronic circuit can be freely changed later.